Dissolution of monazite. The first step in all of the sulfuric acid processes is dissolution of monazite. The procedure recommended by workers at Iowa State [Bl] is as follows. Monazite ground to minus 65 mesh is digested with 93% sulfuric acid for 4 h at 210°C in a stirred reactor. The mass ratio of acid to sand, based on 100% H2S04, is 1.56. The temperature must be kept below 230°C to prevent formation of water-insoluble ThP207. The monazite is converted into a thick paste soluble in cold water. The reaction mass is cooled to 70 C and diluted with about 10 kg cold water/kg monazite. Most of the thorium, rare earths, and uranium go into solution, leaving a sludge of silica, rutile, zircon, and some unreacted monazite. Most of the solution is decanted from the silica sludge and unreacted monazite. The denser monazite is separated from the sludge and recycled. The sludge is filtered and washed to recover additional solution.

Radium in the monazite may be removed with the sludge by adding barium carbonate before decantation. This forms barium sulfate, which removes radium as insoluble radium sulfate.

This process produces a solution of thorium, rare earths, and uranium cations with sulfate and phosphate anions.

Thorium recovery processes. Because of the many elements in the solution, their chemical similarity, and the presence of phosphoric acid, separation of thorium from this acid solution has proved to be difficult. Wylie [W5] has reviewed the numerous separation processes that have been developed. Figure 6.5 shows the principal steps in seven of these processes and gives references for more details. Processes 4 and 6 appear to be the most economic when thorium, rare earths, and uranium all are to be recovered. Process 4, involving separation of thorium and rare earths from phosphate and uranium by precipitation with oxalic acid, is described next. Process 6, involving separation by solvent extraction with organic amines, is described in Sec. 8.6.

Precipitation with oxalic acid. Figure 6.6 shows the principal steps in the process for separating the sulfuric acid solution of monazite into a thorium concentrate, a rare earth concentrate, and a uranium concentrate developed at the Ames, Iowa, Laboratory of the U. S. Atomic Energy Commission [В1].

The solution of monazite in sulfuric acid containing about 50 to 60 g of thorium and rare earths per liter is diluted with about 4.5 volumes of water and brought to a pH of 1.5 by addition of NH4OH. Oxalate ion is added in the form of recycle sodium oxalate, plus sufficient oxalic acid in 10% aqueous solution to provide 110% of the oxalate ion needed to precipitate thorium and rare earth oxalates. The precipitate is filtered and washed with 1 % oxalic acid in 0.3 N nitric acid. A clean separation of uranium from rare earths plus thorium is claimed.

Because of the comparatively high cost of oxalic acid, economics requires recovery of oxalate ion. This is effected by digesting the thorium and rare-earth oxalates with a
stoichiometric equivalent of sodium hydroxide at 95°C for 1 h, to convert the precipitate to hydroxide, which is filtered and washed with hot water. Oxalate ion is recovered as sodium oxalate, of which 95% is recycled.

Uranium is recovered from the sulfate and phosphate filtrate by anion exchange (Chap. S).

Thorium and rare earths in the hydroxide precipitate are dissolved in nitric acid and separated by solvent extraction with TBP (Sec. 8.7).